The innovation relates to a device to test the runoff behavior from yarn packages of certain yarns intended especially for the weaving sector, comprising a variably driven godet to draw off the yarn from the spool, on which godet the yarn is given a 360.degree. turn, a suction jet mounted in the yarn travel direction behind the godet and an on or off yarn tension located between spool and godet.
A similar device is described, for example, in the periodical "Textile Praxis International", June 1975, p. 687 and following in the paragraph "Optimale Wicklungen fur Texturgarne" by P. Dubach. With this known device which is intended to aid the formation of yarn packages for weaving textured yarns, it is possible to simulate draw-off processes at rates of up to 1500m/min., either continuously or discontinuously (like the weaving process). The yarn draw-off element of this known device is composed of a pair of rollers including a bottom roll (godet) rotating at constant circumferential speed and an upper roll mounted on the armature of a vibrator, which applies pressure steadily (continuous draw-off) or intermittently (discontinuous draw-off) to the bottom roll.
Due to the clamping force of the two rollers, the yarn is pulled through in the direction of rotation. For a continuous draw-off, the yarn is given one turn (360.degree. C) on the bottom roll, to insure absolutely slippage-free pull-through. The suction jet is used to pick up the yarn, otherwise the yarn to be drawn off would wrap around the roller. During this continuous draw-off, the armature with the upper roller is imparted an up and down movement at adjustable cycles by means of an oscillator with amplifier. Thereby, the upper roller is forced down on the bottom roller for the duration of a square wave impulse and together with the yarn between said rollers is accelerated to the circumferential speed of the bottom roller (godet). The yarn tension located between godet and spool grips the yarn during the ascending movement of the upper roller so that it will actually assume a velocity 0 during the stop phase.
The known device presents considerable drawbacks, especially in discontinuous operation when used as filling simulator. Reduction of the yarn velocity to 0 or rapid acceleration of the yarn to the circumferential speed of the bottom roller (godet) poses problems, especially at higher on and off frequencies. Moreover, the known device allows draw-off speeds of only up to 1500 m/min.
The objective of this innovation is the further development of a device of the above described type to the extent that even at high on and off frequencies of 20 to 30 sec.sup.-1, reduction of the yarn velocity to 0 or rapid acceleration of the yarn to the circumferential speed of the godet would be insured. Furthermore, the device according to the innovation should allow draw-off speeds to about 2500 m/min.
This objective is achieved according to the innovation by means of a changeover switch connected with the suction jet and the pneumatically actuated yarn tension.
In the device of the invention, the godet -- as in the known device -- runs at an adjustable but constant circumferential speed and the yarn is given one turn on it (angle of contact about 360.degree. C). When the suction jet is actuated, an instantaneous "rope friction effect" or Servo effect sets in, accelerating the yarn practically without perceptible delay to the circumferential speed of the godet, at which speed it is withdrawn from the spool. The draw-off force of the godet (S.sub.2) is always proportional to the compressed-air-adjustable draw-off force of the suction jet (S.sub.1). The following correlation exists between S.sub.1 and S.sub.2 (Eytelwein formula): EQU S.sub.2 = S.sub.1 . e .sup..mu. .alpha.
wherein
e = 2.781 (natural logarithm base) PA1 .mu. = coefficient of friction between yarn and godet PA1 .alpha. = angle of contact of the yarn on the godet e.g. = 2 .pi. at an angle of contact of 360.degree..
Since the maximum draw-off force S.sub.2 can be adjusted by means of the compressed air actuating the suction jet, it is possible for the first time to have a draw-off device with an accurately adjusted draw-off force, which force can be substantially smaller than the breaking force of the material to be investigated, whereby the precision of the yarn travel is nevertheless insured. It is thus possible to locate on a yarn package winding defects or impairments interfering with unwinding, since the yarn being drawn off does not necessarily break nor does the winding interference yield to force, but rather the yarn travel is interrupted so that the cause of the malfunction can easily be detected on the yarn package.
As the suction jet is actuated, the yarn tension is turned off, i.e., deactivated, so that the yarn reaches the godet without tension. When the suction jet is deactivated, the changeover switch immediately actuates the pneumatically controlled yarn tension. The yarn is thereby efficiently decelerated to 0.
Since, in principle, commercial yarn tensions are not used to decelerate the passing yarn to 0 in a minimum of time, the innovation proposes a yarn tension making possible an especially rapid tensioning of the yarn. This preferred version of a yarn tension comprises a first, stationary tension disc and a second pneumatically actuated tension disc moving in contact with the former. The innovation is explained on hand of enclosed drawings.